**3.2 Organophosphates**

These are a group of synthetic compounds produced by the reaction of alcohols and phosphoric acid. These inhibit the enzyme acetylcholinesterase (AChE), which is responsible for the degradation of acetylcholine. The organophosphate binds to the enzyme, causing it to undergo a conformational change at its binding site to acetylcholine [11]. Their application is mainly by three methods: residual spraying, space spraying and to a lesser extent as larvicides. The organophosphates are extensively used in Southeast Asia, followed by the Americas and the Western Pacific.

**Figure 1.**

*Insecticides used for indoor residual spray (IRS), insecticide treated nets (ITN) and long-lasting insecticide treated nets (LLIN) [9, 11–14].*

#### **3.3 Carbamates**

These are esters of carbamic acid and structurally and mechanistically similar to organophosphate (OP) insecticides. These insecticides work by inhibiting AChE and are commonly used to control agricultural pests. Compared with the other classes of insecticides, the use of carbamates is limited, and mainly used for residual spraying in the African Region [12].

#### **3.4 Pyrethroids**

These compounds are organic, and similar to the naturally occurring pyrethrins produced by the flowers of pyrethrums. On the basis of their biological response, they are divided into two groups – Type I and Type II pyrethroids. Pyrethroids are used in all the four major methods of application: about 70%for residual spraying, 25% for space spraying, and the remainder for treatment of nets and larvicidal purposes. In terms of the weight of active ingredient, pyrethroids are not the most used insecticides, but in terms of spray coverage they are by far the most used insecticides. The large-scale usage of pyrethroid insecticides for vector control is worrisome because it exerts a high selection pressure for the development of resistance in vector populations. The genes conferring resistance against pyrethroids have been spreading in vector populations, particularly in the populations of malaria and dengue vectors [13, 14]. This is particularly concerning because the use of long-lasting insecticidal nets (LLINs), a major tool in malaria control, depend solely on the action of pyrethroids. It is critical that the susceptibility of malaria vectors to pyrethroids is preserved. Therefore, it has been recommended to not use pyrethroids for indoor residual spraying where there is a high coverage of its use with treated nets [13].

#### **3.5 Insect growth regulators (IGRs)**

These are diverse group of chemical compounds, the use of which dates back to 1980s [15]. These compounds are effective against the larval stages of insects. They are divided into juvenile hormone analogs and chitin synthesis inhibitors. They mimic insect hormones, such as juvenile hormone and ecdysone, and interfere with the normal growth and development of the insect.

#### **3.6 Bacterial larvicides**

These larvicides are based on the bacteria of the species *Bacillus sphaericus* (Bs), and *Bacillus thuringiensis* serovar *israelensis* (Bti), which are entomopathogenic. In early 1960s the first strain of *Bs* with its larvicidal activity was discovered but in 1976 the Bti subspecies was discovered which was highly toxic to larvae of many species of mosquitoes. By mid-1980s, the use of bacterial larvicides started in different vector control programmes.

#### **4. Insecticide resistance in disease vectors**

The insecticide act in many different ways, to which insects have also developed different mechanisms by which they develop resistance against the toxic effects of these insecticides. Broadly, there are three different mechanisms of development of resistance, i.e., metabolic resistance, target site and resistance to penetration of

the insecticide. Correspondingly, different biological, biochemical and molecular methods have been developed to detect these mechanisms in different vectors.

Status of insecticide resistance in vectors of parasitic diseases
